IGRT is the process of frequent two and/or three-dimensional imaging, during radiation treatment, used to direct the delivery of the therapeutic radiation.
It is known that exposure of human or animal tissue to ionising radiation will damage the cells thus exposed. This finds application in the treatment of pathological cells, for example. In order to treat tumours deep within the body of the patient, the radiation must however penetrate the healthy tissue in order to irradiate and destroy the pathological cells. In conventional radiation therapy, large volumes of healthy tissue can thus be exposed to harmful doses of radiation, potentially resulting in unacceptable side-effects. It is therefore desirable to design a system for treating a patient with ionising radiation and treatment protocols so as to expose the pathological tissue to a dose of radiation which will result in the death of those cells, whilst keeping the exposure of healthy tissue to a minimum.
Several methods have previously been employed to achieve the desired pathological cell-destroying exposure whilst keeping the exposure of healthy cells to a minimum. Many methods work by directing radiation at a tumour from a number of directions, either simultaneously from multiple sources or multiple exposures over time from a single movable source. The dose deposited from each direction is therefore less than would be required to destroy the tumour, but where the radiation beams from the multiple directions converge, the total dose of radiation is sufficient to be therapeutic. By providing radiation from multiple directions, the damage caused to surrounding healthy cells can be reduced.
Intensity modulated arc therapy (IMAT) is one method of achieving this, and is described in U.S. Pat. No. 5,818,902. In this process, the radiation source is rotated around the patient, and the radiation beam collimated to take a desired shape depending on the angle of rotation of the source, usually with a multi-leaf collimator (MLC). The potential advantages of a particular form of IMAT, volumetric modulated arc therapy (VMAT), have recently given rise to a number of commercial implementations and research studies. In these systems, the dose rate, rotation speed and MLC leaf positions may all vary during delivery. In general, plans comparable in quality and accuracy to static-gantry intensity-modulated radiotherapy (IMRT) can be obtained, normally with reduced delivery times.
In typical IMRT methods, a linear accelerator rotates on a gantry around the patient, emitting “modulated” beams of X-rays from a number of pre-fixed angles, where modulation is carried out using a multi-leaf collimator (MLC) attached to the head of the linear accelerator. The MLC shapes the pattern of the outgoing radiation beam, through a sequence of movements of its metal leaves, in order to precisely target the tumours while minimizing exposure of the neighbouring healthy structures.
To make sure the radiation beams are correctly directed, the treatment can be guided by imaging of the target region, before or even during a course of radiation treatment—although the latter is usually predicated on a system where a course of treatment is divided into individual treatments (called “fractions”, where a treatment is applied on a single day, for example), and imaging is carried out between fractions. This is known as IGRT; a typical IGRT method might include localization of a cone-beam computed tomography (CBCT) dataset with the planning computed tomography (CT) dataset from planning. IGRT might also include matching planar kilovoltage (kV) radiographs or megavoltage (MV) images with digital reconstructed radiographs (DRRs) from the planning CT.
Kilovoltage computational tomography (CT) is carried out during treatment by providing a separate source of imaging radiation mounted on the rotatable gantry, placed at an angle relative to the main radiation head. A detector is positioned diametrically opposite the source of imaging radiation, and collects imaging data for a plurality of rotational angles of the gantry. This data can then be reconstructed to form three-dimensional images using known CT techniques. See PCT application WO 2006/030181 for an example of this method. Kilovoltage radiation is often preferred for imaging due to the high contrast between different structures in the patient.
In megavoltage computational tomography (CT), a radiation detector is placed on the rotatable gantry diametrically opposite the main treatment head, and is designed to detect the megavoltage radiation after it has passed through (and been attenuated by) the patient. The images generated are therefore individual transmission images, from the beam's eye view (BEV). Megavoltage imaging can be used to verify the position of the MLC leaves in relation to the target within the patient. The detector is usually known as an ‘electronic portal imaging device’ or EPID. However, the high energy associated with therapeutic radiation is not ideal for imaging purposes as the attenuation coefficients of the various tissue types within a patient are similar at this energy level, leading to poor image contrast. In addition, this method is inherently two-dimensional because in conventional radiotherapy the megavoltage beams are directed at the patient from typically at least three angles, which may be insufficient to provide three-dimensional imaging.
The above two methods comprise the majority of IGRT strategies currently employed. However, radiation therapy systems which incorporate real-time MRI tracking of tumours or other radiation targets are currently being developed. One problem with existing and planned radiotherapy systems is to ensure that the radiation distribution is applied accurately according to the treatment plan, in terms of both the locations the radiation is delivered to and the amount of radiation delivered (the “dose”) to any particular location. A second problem is that of beam-angle optimisation, which is to determine the “optimal” number and values of gantry angles, which is often formulated as a combinatorial optimisation problem. Another, interrelated problem is to reduce the amount of radiation, or dose, applied to non-targeted tissue which is adjacent to targeted tissue and/or in the path of radiation beams applied to targeted tissue. In addition, each treatment should be delivered in a short time, to minimise the effects of patient or target movement and also to maximise the use of the radiotherapy system. These non-trivial problems are rendered even more complex by practical issues such as the characteristics and/or limitations of the radiotherapeutic and the imaging systems, the accurate positioning of the patient and the target tissue before treatment and the possibility of there being movement of both of these, both inter- and intra-treatment, and inter- and intra-fraction. Computerised treatment planning systems attempt to address these problems, however the computational methods and algorithms used are extremely complex and involve enormous amounts of data to be manipulated, which requires large amounts of processing and takes a significant amount of time. Research continues into methods of delivering radiation in such a way that the dose distribution is accurate (i.e. ensuring that the radiation is delivered to the intended locations, or target region(s), and not to other locations) whilst ensuring careful control of the absolute dose delivered to any single location (i.e. ensuring that the amount of radiation delivered to a target region is in accordance with the treatment plan—at or up to a certain level in the case of a tumour, below a predetermined safe level in the case of non-targeted but non-sensitive tissue, and at a negligible level in the case of certain sensitive, non-targeted tissue).
In the IMRT planning process there are further problems which also need to be considered, all to do with optimisation. One is called the fluence optimisation problem, which is to find a set of “optimal” intensity profiles corresponding to the given set of beam angles. Another is the leaf sequencing problem which is to determine an “optimal” sequence of MLC leaf movements that delivers the intensity profile for each beam angle.